The HD598 should be a little easier for the smartphone/mp3 player to drive, but I would assume the KXX has (at least a little) a more likeable sound, but you might consider getting a FiiO E11K headphone amplifier ($60), to daisy chain of the smartphone/mp3 player, as the AKGs usually like a decent amount of power to sound their best.
Might also try asking about headphones on Head-Fi website.